1. FIELD OF THE INVENTION
This invention relates in general to the construction of plasticizing devices and in particular to a new and useful plasticizing device for an injection molding machine which includes a plasticizing screw having a plurality of axially extending and circumferentially spaced passages around its periphery adjacent the cylindrical housing discharge thereof.
2. DESCRIPTION OF THE PRIOR ART
The present invention relates in particular to a plasticizing device of an injection molding machine for plastics including an axially displaceable plasticating screw and a check valve, comprising a seat body positioned in the plasticating cylinder as well as a seal ring which axially cooperates therewith and is fixed to the screw.
In known plasticating devices of this kind, granulated plastic material is fed into the plasticating cylinder and it is plasticized in the zone of the screw thread of the rotating screw, while the homogenization, i.e. the intimate mixing and the plastication of the perhaps not yet completely plasticized particles of the granulate takes place in the zone of the discharge end portion of the screw in a discharge or metering zone.
With small quantities of plastic fed through or discharged, the period of dwell of the plastic in the discharge or metering zone of the plasticating screw is sufficiently long to permit transferring to the plastic of enough thermal energy, by heat input from the outside as well as by the produced frictional heat, necessary for a completely satisfactory homogenization. In addition, the optimum homogeneity of the plasticated plastic can be controlled by varying the back pressure directed against the screw.
With large quantities of plastic fed through or discharged, however, such as are needed, for example, for the manufacture of thin-walled packaging parts, and if usual screws are used having a small length-to-diameter ratio, the limit of a completely satisfactory homogenization and intimate mixing of the plastic, for example, with dry or wet coloring substances, is reached very soon, i.e. non-melted particles of the granulate and an irregular distribution of the colorant occur in the molded packaging part.
Up to date, these drawbacks could be avoided only by using special screws having a larger length-to-diameter ratio. Known are already so-called static mixing systems which are mounted on the plasticating cylinder at the end of the discharge or metering zone of the plasticating device, thus forming a connecting member between the plasticating device and the tool. Such systems, however, do not provide a nozzle closure so that, in order to avoid a leakage of material disturbing the cycle, it becomes necessary to provide the metering and the relieving of the plastic melt in the plasticating unit in a period of time prior to the tool opening. Further known are co-rotating mixing elements which, instead of the screw threads, comprise cam-like extensions of various kind the shearing effect of which produces heat in the plastic melt. These devices involve a strong increase in the needed torque or drive power. For extreme performances in plastication, these devices do not prevent the escape of non-plasticized particles of material either. In addition, these mixing systems are very expensive.